so I got asked a question about getting gas from two different gas stations, where brand a got 4 more miles to the gallon then brand b did but brand b costs less. I don't know the question so here is is and I'm hopping one of you will have an answer.

Is the gas that the car is getting a higher mpg on easier on the engine in terms of will it take longer to wear out any of the parts than using the other kind of gas? given that we aren't going to drive the car any differently either way and it is a fairly modem engine (2008)
and neither gas station is out of the way so we aren't putting any extra miles to get the gas, and both gases are advertised as having the same octane rating.

is it just down to something as simple as we might have to change a filter more often or could have bigger effects?

Why do you think that brand a gas returns better mileage than brand b gas? Brand a's advertising? Is that really a trustworthy source of information?
– DavidSupportsMonicaApr 25 '19 at 5:42

it's not advertising, it's from me tracking what my car is saying it's getting, as in I looked at was it was getting when it was using brand a and then what it was getting when it was filled with brand b, then calculated the difference and I did reset the tracker in the car in between fill ups.
– Kit RamosMay 3 '19 at 15:32

To be accurate, the drives should have been exactly identical. Commuting in the city consumes a lot more fuel than cruising on the highway.
– Pierre P.May 6 '19 at 8:28

@PierreP. I did drive the same route on both tests. so that wouldn't be a factor.
– Kit RamosJan 28 at 18:09

2 Answers
2

The difference in gas mileage can be attributed to the ethanol content in the gas. Ethanol has a few properties that make it common in today's gas stations: it's supposedly more "environmentally friendly" and sustainable, it typically has a much higher octane rating so gas companies can use it to boost low octane gas, and it's a lot cheaper. Because of this, you'll often see gas pumps that list that their gas is a 10% or 15% ethanol blend. This can result in an observable difference compared to a pure gasoline product.

However, ethanol also has a drawback: it has lower energy density than gasoline, so more fuel is required to produce the same power, and thus the gas mileage is worse (in terms of MPG).

At the end of the day, this alone isn't enough to say whether the quality of the gas at one station is better than the other. Chances are, if they're both reputable companies, then it'll be safe to run the gas from either gas station (disclaimer: don't take my word for it, and research the companies for yourself). What you can do is compare the difference in gas mileage you get between the two pumps, and then compare that with the difference in price, and determine which gas station gives you better mileage per $ spent (you can also divide MPG by $/gal).

thanks for the info, that's what I was thinking might be the case. I figured I could math out the price difference, but wasn't sure if there was other factors in play but sounds like the answer is, probably not, as both are big reputable brands. Shell was the one that got the extra distance and Speedway was the cheaper one. Btw cute name and avatar.
– Kit RamosMay 3 '19 at 15:39